Study the behaviour of citric acid with regard to several substances dissolved in water and acetone.

Transcription

1 Science - Chemistry - Acids, Bases, Salts - Acids (P758500).6 Brönstedt acids - comparison of the acidity of an aqueous and an acetonic citric acid solution Experiment by: Phywe Printed: Oct 5, 203 :43:4 PM intertess (Version 3.06 B200, Export 2000) Task Task What is the importance of water for acid solutions? (2) Study the behaviour of citric acid with regard to several substances dissolved in water and acetone. Use the space below for your own notes. Logged in as a teacher you will find a button below for additional information. - -

2 Additional information Learning objectives Acids show typical properties like the coloration of indicators or the reaction with base metals. However, substances that can form an acid only show these properties in the form of an aqueous solution. Notes on set-up and procedure Preparations The acetone used for this experiment must be completely free from water. Acetone that has already been stored for a longer period of time must be dried for at least 24 hours over anhydrous calcium chloride. Use some nippers or a similar tool to cut off small pieces of marble which fit into the test tubes. Remarks on the students' experiments It is recommended to use rather small pieces of marble and short pieces of magnesium ribbon since otherwise the reaction would takwe too long. Hazard and Precautionary statements Magnesium, ribbon: H228: Flammable solid. Citric acid: H39: Causes serious eye irritation. P305 + P35 + P338: IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing

3 Acetone: H225: Highly flammable liquid and vapour. H39: Causes serious eye irritation. H336: May cause drowsiness or dizziness. EUH066: Repeated exposure may cause skin dryness or cracking. P20: Keep away from heat/sparks/open flames/hot surfaces. - No smoking. P233: Keep container tightly closed. P305 + P35 + P338: IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing. Hazards Acetone vapours are hazardous to health. Do not inhale them! Put on protective glasses! Notes Acetone (propanone) as a polar-aprotic solvent has only small solvating power. It is a very poor proton acceptor so that the citric acid which is a weak acid anyway does not show any acid properties when being dissolved in acetone. Remarks on the method This experiment leads directly to the Brönstedt theory the fundamental principles of which have been worked out in the first experiment. As far as the answer to question 3 is concerned, the students need to know the concept of ions. However, this experiment together with the preceding one is also appropriate to work out and introduce the ion concept since the conclusion to be drawn from the conductivity determined in this context is the existence of charged particles. Before acids can be finally defined as proton donors, an acid must be submitted to an electrolysis in order to show that the hydrogen ions in an aqueous solution of an acid are present in the form of protons. It is recommended to carry out the electrolysis of hydrochloric acid in the course of a demonstration experiment since in this case the "acid residue" chlorine is formed at the anode. Waste disposal Let the aqueous citric acid solution react completely and put it into the collecting tank for acids and alkalis. Put the solution of citric acid in acetone into the collecting tank for organic solvents

4 Material Material Material from "TESS Chemistry Set Acids, Bases and Salts" (Order No ) Position No. Material Protective glasses, clear glass Order No Quantity 2 Test tube rack for 2 tubes Graduated cylinder, 50 ml, PP Laboratory pencil, waterproof Spatulas, double bladed, l = 50 mm, bent Tweezers, straight, blunt, 30 mm Beaker, 00 ml, short, plastic, stackable Litmus paper, blue, box Test tube, 8 88 mm, 0 pcs (6) Chemicals, additional material Position No. Material Acetone, 000 ml Order No Quantity Citric acid, 250 g Magnesium, ribbon, 25 g Marble, pieces, 000 g Tap water Material required for the experiment - 4 -

5 Setup Set-up Hazards Acetone vapours are hazardous to health. Do not inhale them! Put on protective glasses! Set-up Number the test tubes from to 6 and put them into the test tube rack (Fig. ). Fig. Fill 20 ml of acetone into one of the beakers (Fig. 2) and add two spatulas of citric acid (Fig. 3). Let the acid dissolve. Fill 20 ml of water taken from the second graduated cylinder into the second beaker. Add two spatulas of citric acid and let it dissolve. Fig. 2 Fig

6 Action Action Procedure Distribute the citric acid dissolved in acetone equally to the test tubes to 3 and the citric acid dissolved in water to the test tubes 4 to 6 (Fig. 4). Fig. 4 Put a strip of blue litmus paper into test tube (Fig. 5), a piece of magnesium ribbon into test tube 2 (Fig. 6) and a piece of marble into test tube 3 (Fig. 7). Fig. 5 Fig. 6 Fig. 7 Put these substances in the same order into the test tubes 4 to 6 (Fig. 8)

7 Fig. 8 Waste disposal Let the aqueous citric acid solution react completely and put it into the collecting tank for acids and alkalis. Put the citric acid dissolved in acetone into the collection tank for organic solvents

8 Evaluation Evaluation Question : Enter your observations into Table. Table litmus paper magnesium marble citric acid in acetone citric acid in water Table a litmus paper magnesium marble citric acid in acetone no change no reaction no reaction citric acid in water turns red magnesium dissolves while issuing a gas marble dissolves while issuing a gas - 8 -

9 Questions and exercises Questions and exercises Question : Draw the conclusions from your observations. The solution of citric acid in acetone does not react with any of the added substances. Unlike the aqueous citric acid solution it does not show any of the typical properties of acids like the coloration of an indicator or the dissolution of base metals. Question 2: Summarize the conclusions in the form of a catchword. Substances that can form an acid only show the typical properties of an acid in the form of an aqueous solution

10 Question 3: What is the role of the water in the formation of an acid solution? Which ions are generated in this context and in what way could their charge be determined? Similar to salts, acid molecules, too, are split up into ions due to the influence of the water which can be shown by means of a conductivity measurement. During this process hydrogen ions must be formed since base metals only react in aqueous solutions while at the same time issuing hydrogen. The charge of the hydrogen ions (acid residue anions) can be determined by means of an electrolysis. Certain ions would move towards the cathode, others towards the anode